1. Field of the Invention
This invention relates to a disconnectable power semiconductor component, and more particularly to a field-controlled thyristor of the type which is described, for example, in publication DE-A No. 2,855,546.
2. Discussion of Background
Field-controlled power semiconductor components are known as having different structures and by various names. Within these known components, two functional types are mainly distinguished, which are unipolar structures such as, for example field-effect transistors of the MOSFET or JFET type having majority carrier conduction, and components having bipolar carrier injection such as, for example the FCT (Field Controlled Thyristor) or the SITh (Static Induction Thyristor).
For applications in the high performance field, especially the last-mentioned bipolar structures, the action of which is explained in greater details in the above-mentioned publication and in DE-A No. 3,002,526, are of interest for physical reasons.
DE-A No. 2,932,043, DE-A No. 2,824,133, or IEEE Transactions ED-29 (1982) 1560, or IEDM Technical Digest 1984, 439, and EP-A No. 0,121,068 are also relevant publications.
As a rule, the action of the conventional structures is based on applying the JFET principle for control: in finely distributed gate or control zones, application of a negative gate voltage generates areas with charge carrier depletion, which, with rising gate voltage, extend into a channel region which conducts the current and finally interrupt the current flow by pinching off the channel region.
The pinching-off process is counteracted by the anode voltage present at the component so that with increasing anode voltage, an increasing gate voltage is also required for blocking or switching-off the component. The ratio between the anode voltage and the gate voltage needed for blocking is called turn-off gain.
There is a close connection between the turn-off gain and the geometric arrangement and design of the gate zones, which results from the fact that for blocking a certain voltage by means of the gate voltage applied, a "penetration" through the field-controlled channel must be prevented.
In the blocking state, the positive space charge in a JFET structure with n-doped channel produces a positive curvature of the associated potential surfaces. However, to prevent an injection of electrons from the cathode-side n.sup.+ emitter into the channel, the potential must not become greater than zero at this point. This can be achieved if it is possible to make the curvature of the potential surface negative in the axial direction in the channel.
For this purpose, a correspondingly large positive curvature component must be generated in the lateral direction by the negative gate voltage at the p.sup.+ gate regions in the known components. But the amount of this component is essentially determined, in addition to the gate voltage, by the difference of the adjacent gate regions. It follows from this that in a structure of the known type an increase in the turn-off gain can be achieved in principle only by reducing the distances between the gate regions, that is to say by an even finer subdivision of the gate-cathode structure.